1. Field of the Invention
The present invention relates to novel acid systems that are capable of stabilizing high concentrations of tertiary cations and forming carbonium ion salts containing dimeric and monomeric anions. The ions undergo hydride and halide exchange with other alkanes and halides. The hydride transfer reaction renders them useful in isomerization and alkylation reactions.
2. Description of the Prior Art
Isomerization of a gasoline fraction to increase branching is one of the simplest of the common reactions of petroleum chemistry. Model studies have been carried out with butane, pentane and simple alicyclics that yield relatively uncomplicated product mixtures. For example, the butanes may be equilibrated fairly rapidly in contact with aluminum halides at temperatures of 100.degree. C. or higher.
It is known that pure dry aluminum bromide is not an effective catalyst unless the system contains some trace of alkyl halide, alcohol, or a combination of an alkene and a proton source. Examples of aluminum bromide promoted isomerization catalyst systems are disclosed in U.S. Pat. Nos. 2,963,526; 2,987,563; 3,641,185; 3,758,623; and 3,946,088. Some of these patented catalyst systems include halogenated compounds, e.g., poly-halogenated benzenes and they are preferably promoted by promoters such as hydrogen halides, alkyl halides and water.
Another example of an isomerization catalyst is described in U.S. Pat. No. 3,725,500, the disclosure of which is incorporated herein by reference. In this patent, the catalyst comprises a mixture of aluminum halo bromide and a sulfur oxo halide, e.g., AlBr.sub.3 /SO.sub.2 FCl mixtures. The catalyst composition is capable of stabilizing high concentrations of alkyl carbonium ions and it is useful in isomerization reactions.
The promoters in these catalyst systems lead to the formation of carbonium ions which initiate chain reactions leading to rearrangement of the hydrocarbon feedstock. A key step is the hydride transfer reaction involving hydrogen transfer from a hydrocarbon to a carbonium ion. Rearrangement of the new carbonium ion leads to isomerization.
The following equations illustrate these reactions:
Initiation: EQU (CH.sub.3).sub.3 CBr + AlBr.sub.3 .revreaction.(CH.sub.3).sub.3 .sup.+AlBr.sub.4 EQU (Promoter)
Propagation:
(a) Hydrogen transfer - Bartlett-Condon-Schneider hydride transfer between n-butane and the tert.-butyl carbonium ion: EQU (CH.sub.3).sub.3 C.sup..sym. + CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.3 .revreaction.(CH.sub.3).sub.3 CH + CH.sub.3.sup..sym. CHCH.sub.2 CH.sub.3
(b) Rearrangement of the sec.-butyl carbonium ion to the tert.-butyl carbonium ion: ##STR1##
(C) Propagation step: ##STR2##
Conversion of n-butane into isobutane is of importance industrially, and is interesting as a prototype for catalytic isomerization of low-octane gasolines. Considerable amounts of n-butane are produced in cracking reactions and converted to isobutane, which is needed for synthesis of high octane blending components of motor gasoline by alkylation of olefins.
It is further known that high-octane gasoline can be obtained by catalytic recombination of C.sub.2 to C.sub.5 olefins and isoparaffinic hydrocarbons. The reactants in the alkylation process are generally contacted in the liquid phase at temperatures usually below about 100.degree. F., although on occasion, higher temperatures may be utilized and at pressures varying from ambient to superatmospheric. The alkylation process is generally carried out in the presence of acidic catalyst such as sulfuric acid or liquid hydrogen fluoride as shown below: ##STR3## An example of an acid-catalyzed alkylation process is described mole U.S. Pat. No. 3,231,633.
The alkylation reaction has been regarded as proceeding through the acid catalyzed, addition of a t-butyl cation to butylene followed by a hydride transfer from isobutane. The latter reaction is favored in competition with the formation of higher isobutene polymers by use of excess alkane in the feedstock.
G. A. Olah et al. (J. Am. Chem. Soc., 85, 1328 (1963) and J. Am. Chem. Soc., 86, 1360 (1964) and D. M. Brouwer et al. (Proc. Chem. Soc., 147 (1964)) have reported that acid systems stronger than H.sub.2 SO.sub.4 or HF have the ability to stabilize tertiary and in some cases secondary cations for a sufficient time to permit their observation by NMR spectroscopy. In principle this should permit the study of ionic equilibria engendered by the fast intermolecular hydride transfer reaction between the ions and alkanes that was studied under other conditions by Bartlett, Condon and Schneider (J. Am. Chem. Soc., 66, 1531 (1944)).
Typical acids that permit the detection of tertiary cations have been characterized in terms of the H.sub.o or Hammett acidity function. Suitable mixtures in HSO.sub.3 F (i.e., SbF.sub.5 + HSO.sub.3 F) have H.sub.o 's greater than -16. Hammett acidity values for HF systems also have to be about this high. Typical HF systems involve one molar solutions of TaF.sub.5, NbF.sub.5 or SbF.sub.5 in HF. The acidities of these solutions is much greater than HF itself (H.sub.o = -11), but these solutions are not generally useful for studying hydride transfer equlibria by NMR spectroscopy because all soluble alkanes are converted to carbonium ions.